Blow mold for forming a refillable polyester container

ABSTRACT

This relates to a returnable/refillable container in the form of a blow molded polyester biaxially oriented bottle wherein the container includes a domed base construction wherein the wall thickness gradually and smoothly increases to a maximum at the center of the dome. This also relates to a specially configurated preform which is blow molded to form the container and a specially configurated blow mold cavity for receiving a preform having a lower axial projection in the form of an elongated gate without crushing.

This application is a continuation of application Ser. No. 07/758,800,filed Sep. 12, 1991, now abandoned, which is a division of Ser. No.07/488,247 filed Mar. 5, 1990, now U.S. Pat. No. 5,066,528.

This invention relates to new and useful improvements in plasticcontainers, and more particularly to a blow molded polyester containerhaving an improved base construction which is resistant to straincracks, and the preform from which such container is blow molded.

BACKGROUND OF THE INVENTION

An economically and commercially viable, refillable plastic bottlereduces the existing land fill and recycle problems associated withdisposable plastic containers, and more particularly with plasticcontainers formed of PET. In addition, a refillable container willpermit the entry of the safer, lighter weight plastic containers intothose markets currently dominated by glass, where legislation prohibitsuse of non-returnable packages.

The desirability of a refillable container formed of a polyester isspecifically discussed, for example, in U.S. Pat. No. 4,334,627 grantedJun. 15, 1982. The disclosure of that patent is incorporated herein byreference.

At the present, it is to be understood that several polyester candidatesprovide the desired clarity and physical properties deemed necessary toproduce refillable plastic containers. These polymers includepolyethylene terephthalate (PET) acrylonitrile, polyarylate,polycarbonate, etc. Of the polymers commercially available, PET, at thepresent, offers the best balance of properties and cost/performanceratios.

The container specifically disclosed in U.S. Pat. No. 4,334,627 has beenimproved and is specifically disclosed in U.S. Pat. No. 4,725,464,granted Feb. 16, 1988. This patent makes specific reference to U.S. Pat.No. 4,334,627.

This invention relates to an improvement in the container specificallydisclosed in U.S. Pat. No. 4,725,464 and the preform from which suchcontainer is blow molded.

OBJECTS OF THE INVENTION

A refillable container must go through a loop each time it is reused.The loop is comprised of (1) an empty caustic wash followed by (2)contaminant inspection and product filling/capping, (3) warehousestorage, (4) distribution to wholesale and retail locations, and (5)purchase, use and empty storage by the consumer followed by eventualreturn to the bottler. It is the hot caustic wash which is mostdeterminental to the reuse of a blow molded polyester container. It hasbeen found that failure (via crack initiation and propogation) ofbiaxially oriented blow molded polyester containers exposed to thecaustic wash baths occurs primarily in the base area and mostparticularly in the central part of the base area which has little or noorientation. It is to the improvement of the resistance of biaxiallyoriented blow molded polyester containers, particularly PET bottles andthe like, that this invention relates.

In accordance with this invention, the specific configuration of thebase forming portion of the preform has been modified so as to providefor an improved container base construction.

With the above and other objects in view that will hereinafter appear,the nature of the invention will be more clearly understood by referenceto the following detailed description, the appended claims and theseveral views illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic showing the typical cycle or loop to which arefillable container must pass.

FIG. 2 is a vertical sectional view taken through a container of aconfiguration to which this invention specifically relates.

FIG. 3 is an enlarged fragmentary vertical sectional view taken throughthe base construction of the container of FIG. 2.

FIG. 4 is a vertical sectional view taken through a preform for formingthe container of FIG. 2.

FIG. 5 is a horizontal sectional view taken generally along the line5--5 of FIG. 4 and shows the specific cross section of the body portionof the preform.

FIG. 6 is a transverse horizontal sectional view taken generally alongthe line 6--6 of FIG. 4 and shows specifically the increase in thicknessof the container base forming portion of the preform.

FIG. 7 is an enlarged fragmentary vertical sectional view taken throughthe base portion of the preform and shows the specific configuration ofthe improved preform construction.

FIG. 8 is a fragmentary vertical sectional view showing generally thebase construction of FIG. 7 with the bottom central part of the basebeing engaged by a stretch blow centering rod.

FIG. 9 is a fragmentary vertical sectional view taken through a baseportion of a blow mold in which the preform of FIG. 4 is blow molded toform the container of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

First of all, each container is subjected to a typical commercialcaustic wash solution which is prepared 3.5 sodium hydroxide by weightwith tap water. The wash solution is maintained at 140° F. and thecontainers are submerged uncapped in the wash for 15 minutes inaccordance with a commercial bottle wash system.

After removal from the wash solution, the containers are rinsed in tapwater and then filled with a desired liquid product which may be in theform of a carbonated drink which is packaged at 4.0±0.2 atmospheres andcapped. Such a filled container must withstand a temperature of 100° F.at 50% RH for 24 hours.

The general loop through which a container passes in a single use of thecontainer is shown in FIG. 1.

Generally, the degree of molecular orientation in the upper shouldersection and in the base section of a stretch blow molded container islower than that in the body or panel section of such container. In arefillable container, all features such as flutes, grooves and steps,which are potential stress risers, are avoided in the upper shoulderregion and in the base section. Any decorative embossing, flutes,grooves, etc. should be restricted to sections having higher degrees oforientation.

To achieve a good balance between creep-volume growth and thermalshrinkage in the hot caustic wash, the preform segment, which forms thepanel section of the container, is configurated to give a totalthickness reduction ratio of 7.5 to 10.5, with a stretch ratio of 3.2 to3.5 in the hoop direction and 2.3 to 2.9 in the axial direction.

The lower body and base forming portion of the preform (see FIG. 7) isconfigurated to have sections PQ, QR, RS and ST. These form the segmentspq, qr, rs and st, respectively, of the container shown in FIG. 3.

Referring now to the drawings in detail, reference is first made to FIG.4 wherein there is illustrated the vertical cross section of a preformformed in accordance with this invention, the preform being generallyidentified by the numeral 10. The upper part of the preform 10 is of thesame configuration as the preform of U.S. Pat. No. 4,725,464 andincludes a threaded neck finish 12 which terminates at its lower end ina capping flange 14. Below the capping flange 14, there is a generallycylindrical section 16 which terminates in a section 18 of graduallyincreasing external diameter so as to provide for an increasing wallthickness. Below the section 18, there is an elongated body section 20,the extreme lower part of which forms an upper part of a container baseforming portion generally identified by the numeral 22. The base formingportion 22 includes at least the section QR (FIG. 7) at the top thereof.As is best shown in FIG. 7, in the section QR, the internal diameter ofthe preform 10 gradually decreases as at 24 wherein the section RS is ofincreased wall thickness.

It will also be seen from FIG. 7 that the diameter of the section STgradually reduces so as to reduce the wall thickness of the section ST.Additionally, the lower part of the section ST is generally of ahemispherical configuration as at 26. Furthermore, the inner surface ofthe section ST as at 28 is also of a generally hemisphericalconfiguration, but with the surfaces 26 and 28 having different centersso that there is a continued gradual decrease in wall thickness.

At the center T of the section ST, there is a smoothly roundedupstanding projection 30 which is to be received in the lower end of astretch blow centering rod, as shown in FIG. 8 and to be described inmore detail hereinafter. Further, at the bottom of the center T, thereis a downwardly directed axial projection 32 which is in the form of anelongated gate. The outer surface 26 of the preform 10 smoothly turns toform the outer surface of the projection 32.

Reference is now made to FIG. 9 wherein there is illustrated theconfiguration of the base portion of a blow mold cavity 34 defined by acustomary type of blow mold 36. The blow mold cavity 34 in the base ofthe blow mold 36 has a lower outer generally hemispherical surface 38which terminates in a base contact radius C which is centered a distanceB which is 50 to 75% of the general overall radius A of the blow moldedcontainer. The radius C is 5 to 15% of the radius A.

It will be seen that the blow mold 36 is of a configuration so as tohave a central dome 40 which is of a height D having a dimension whichis 20 to 40% of the radius A.

It will also be seen that the dome 40 is provided with a central recess42 of a size and shape to receive the projection 32. Further, the domedsurface of the cavity 34 also includes a surface portion 44 whichgradually slopes into the recess 42.

At this time it is also pointed out that the wall thickness of thepreform at the cross section U in FIG. 7 is about 50 to 70% of the crosssection of the preform in the segment RS.

The preform 10 is reheated to the required glass transition temperature,placed within the blow mold 36 in the conventional manner, engaged by astretch blow centering rod 50, as is shown in FIG. 8, inflated byinternal pressurization so as to be stretched in both the axial and hoopdirection to match the configuration of the cavity 34 and form acontainer such as the container of FIG. 2 which is identified by thereference numeral 52. At this time, it is to be noted that the rod 50has a recess 54 in the bottom thereof so as to clear the projection 30.At the same time, the projection 32 is received in the recess 42. As aresult, when the container 52 is blow molded within the blow mold 36,both the inside surface and the outside surface of the base, generallyindentified by the numeral 56 of the container 52, will be free ofstress concentrations.

The container 52 will include the threaded neck 12 and the capping orsupport flange 14 in their original configurations. Immediately belowthe flange 14, the container 52 will have a generally unoriented thickwall portion 58 which terminates in a thin wall shoulder 60 which isformed from the preform section 18. Beneath the shoulder 60, there isthe container body 62. In the illustrated form of container, thecontainer body 62 is provided with an elongated panel section 64 whichis of a reduced cross section and is intended to receive a suitablelabel (not shown). The body 62 continues below the panel section 64 asat 66 and terminates in the base 56, which, as previously described, isof the domed type.

The wall thicknesses of the various portions of the container 52 abovethe base construction 56 are clearly shown in FIG. 2. At this time it ispointed out that the container 52 is a 1.5 liter bottle and the preform10 is configurated to form the container 52.

Reference is now made to the enlarged showing of the base construction56 found in FIG. 3. It is to be noted that the various lines P, Q, R, Sand T find their equivalent lines in the container 52 as lines p, q, r,s and t, respectively. The base construction 56 starts generally at theline p and includes an axially extending and radially inwardly directedouter surface 68 which is generally hemispherical. It will be seen thatthere is a very slight increase in wall thickness between lines p and qwhile the section q, r has a gradual increase in wall thickness with thebase construction terminating in a chime c which forms the supportradius. In the section r, s, the wall thickness continues to increasewith the base construction 56 being domed as at 72 and the wallthickness continuing to gradually increase until it reaches the point 70which corresponds to the outer edge of the sloping surface 44.Thereafter, the base construction 56 continues to smoothly increase butat a slightly greater rate until the outer surface terminates in theprojection 32 and the inner surface terminates in the projection 30.

To achieve the required wall thickness at chime c of the baseconstruction 56, FIG. 3, the wall thickness of segment RS is about 1.10to 1.30× that of the segment PQ. The transition from thin to thick wallsection is gradual and smooth as described above. This eliminates anyabrupt wall thickness change in the segment qr.

A higher degree of orientation in the dome segment st of the base 56 isachieved by optimizing the base configuration and by the gradualdecreases of the preform wall thickness in the segment ST.

The projections 30, 32 at t insures that a crystalline region, if any,in this area does not weaken the section. As pointed out above, theelongated gate 32 is not crushed during blow molding.

Inasmuch as the base construction gradually increases in wall thicknessas the orientation decreases, it will be seen that the desired strengthrequirement is obtained for the base construction 56 by way of smoothlycurved surfaces which greatly reduce the possibility of stress cracking.

It is believed that 28-30% crystallinity is an optimum level for arefillable PET container which is preferably in the form of a bottlehaving an injection molded threaded neck finish. Further, it is to beunderstood that the preform, and thus the container, may be formed of amultilayer construction including internal barrier layers so as toextend shelf life. The application of such multilayer preforms may beutilized to reduce contaminate absorbtion, if filled with non-foodproducts, and subsequent product contamination after washing andfilling.

Although only a preferred embodiment of the refillable plastic bottleand the preform from which it is formed have been specifically set forthherein, it is to be understood that minor variations may be made ineither the container, the preform or the blow mold in which the preformis blow molded to form the container without departing from the spiritand scope of the invention as defined by the appended claims.

We claim:
 1. A blow mold for blow molding a heated preform to form acontainer having an internally domed base, said blow mold defining acavity including a base forming portion, and said base forming portionhaving a central substantially hemispherical dome portion and, at thecenter of the dome portion, a gradually sloping inverted portion with acentral substantially axial recess matching an intended shape of anoriginal depending axial portion on an intended preform.
 2. A blow moldaccording to claim 1 wherein said recess is in the shape of an axiallyelongated gate.